1. Field of the Invention
The present disclosure relates to semiconductor devices, and more particularly to a horizontal semiconductor device having wiring and electrode pads over active regions.
2. Description of the Related Art
Among semiconductor devices, in particular, III-V nitride semiconductors, in other words, mixed crystals expressed by a general expression of AlxGa1-x-yInyN (where 0≦x≦1, 0≦y≦1), such as gallium nitride (GaN), aluminum nitride (AlN), and indium nitride, are physically characterized by a wide bandgap and a direct transition band structure, which allow the mixed crystals to be applied to short wavelength optical elements. Furthermore, such mixed crystals are applied not only to the short wavelength optical elements. Since these mixed crystals have a high breakdown electric field and a high saturated electron velocity, they have been examined to be applied also to electronic devices.
In particular, a hetero-junction field effect transistor (HFET), which is manufactured using two-dimensional electron gas (2DEG) generated at an interface between an AlxGa1-xN layer (where 0<x≦1) and a GaN layer which are sequentially grown above a semi-insulating substrate by an epitaxial growth technique, has been developed as an high output device or a radio-frequency device. This HFET receives supply of electrical charges caused by polarization effects including spontaneous polarization and piezoelectric polarization in addition to supply of electrons from a carrier supply layer (N-type AlGaN Schottky layer). An electron density of an HFET exceeds 1013 cm−2, which is greater than an electron density of an AlGaAs/GaAs HFET by approximately one digit. As described above, an HFET including a III-V nitride semiconductor is expected to offer a higher drain current density than that of a GaAs HFET, and it is disclosed that there is a III-V nitride semiconductor having a maximum drain current exceeding 1 A/mm (see Non Patent Literature 1). Furthermore, a III-V nitride semiconductor, which has a wide bandgap (for example, GaN has a bandgap of 3.4 eV), has also a high breakdown voltage, and an HFET including such a III-V nitride semiconductor therefore has a gate-to-drain breakdown voltage of 100 V or higher (see Non Patent Literature 1). As described above, electronic devices represented by an HFET including a III-V nitride semiconductor are expected to offer electrical properties of a high breakdown voltage and a high current density, and therefore examined for applications to radio-frequency devices and to devices with a smaller design size and a larger power amount than conventional electronic devices.
As described above, a III-V nitride semiconductor device can reduce its active region in size to approximately one third to one tenth of that in a silicon (Si) semiconductor device. However, the conventional III-V nitride semiconductor devices need a large area for electrode pads for wiring connection, thereby failing to sufficiently reduce their sizes. In particular, in application to a power device in which a large amount of current flows, a diameter of wire and a size of a ribbon which are connected to the electrode pads are desirably large. In order to address this, there is a limit to reduce the size of electrode pads. Therefore, a structure in which electrode pads are formed over active regions, which is called a pad-on-element structure, has been examined (see Patent Literature 1).